Ultra-low carbon steel can be produced in an integrated steel mill by a vacuum decarburization treatment following initial decarburization or refinement of the steel, such as through the basic oxygen steelmaking process (BOF) or through the bottom-blown oxygen steelmaking process (Q-BOP). Steel is refined when oxygen is introduced into the molten metal bath and combines with carbon, removing the carbon as carbon monoxide and lowering the carbon content of the molten bath. In a basic oxygen furnace, oxygen is blown from the top into a molten bath of steel at atmospheric pressure while in the Q-BOP process, oxygen is introduced through tuyeres in the bottom of the vessel and passes upwardly through the bath. Following decarburization, dissolved oxygen is retained in the steel. Subsequently, a vacuum-degassing process, such as the RH (Ruhrstahl-Heraeus) process, is able to utilize the dissolved oxygen in the molten steel under a high vacuum condition for further decarburization.
For the production of ultra-low carbon steel (50 ppm and lower), oxygen is blown for a longer period of time during refinement than for other steel grades, resulting in the carbon content of molten steel at tapping being reduced to a low level of 0.015-0.025% and the dissolved oxygen content being maintained at a very high level on the order of 500-700 ppm. Beginning with this very low carbon and very high dissolved oxygen, RH vacuum-degassing equipment, operating at a vacuum below 10 Torr, is able to decrease the carbon content in the molten steel below 50 ppm (0.005%) in a treatment time of about 20 minutes. As the dissolved oxygen content is increased above the minimum level necessary for decarburization, the higher oxygen content results in a faster oxygen-carbon reaction and, together with the lower initial carbon content, results in a shorter decarburization treatment time. Conversely, if the initial carbon content is higher than 0.025% and/or the initial dissolved oxygen content is less than 500 ppm, the vacuum treatment time must be extended to achieve the ultra-low carbon levels. Unfortunately, for baths having too high a carbon content and/or too low of a dissolved oxygen content, the prolonged treatment time often fails to decarburize the molten steel to a level below 50 PPM, acting merely to increase production time.
When a Q-BOP is used for refinement, more efficient use is made of the oxygen for decarburization, resulting in the dissolved oxygen content of the refined steel being lower than that of steel produced by a BOF. Therefore, steel produced in a Q-BOP for subsequent decarburization in a vacuum-degasser may require the addition of oxygen for decarburization to ultra-low levels.
One solution to both of these situations has been the use of an RH-OB treatment. The RH-OB vacuum-degassification system employs tuyeres in a vacuum refinement section for the introduction of oxygen into the steel, assisting decarburization. When oxygen is not required, an inert gas, such as argon, must be delivered through the tuyeres to prevent plugging of the tuyeres during degassing. The argon blown into the RH-OB vacuum chamber acts as a coolant and results in the formation of a solidified metal shell, commonly referrred to as a "skull", in the vessel which must be removed as often as every two to three days and which requires two to three days for removal before the vessel can be reused, causing delays in availability, reducing refractory life and resulting in high operating costs. To circumvent the loss of vessel availabililty during deskulling, the vessel with the skull is moved to a maintenance position and a second vessel is moved into the operating position. This equipment configuration is a more expensive facility than a single vessel facility. Consequently, RH-OB vacuum-degasser with a quick vessel exchange practice is much more expensive than a single RH vacuum-degasser in equipment cost.
What is needed is a method of adding controlled amounts of dissolved oxygen to a molten bath of steel without directly adding gaseous oxygen.
The invention is a method to promote the decarburization reaction in the vacuum refining section of an RH vacuum-degasser by employing the controlled addition of manganese ore without the operating problems that result from the direct addition of gaseous oxygen and argon. The added manganese ore is melted at a high temperature to release oxygen into the molten steel. The manner and quantity of addition are adjustable to the reaction requirement for which oxygen is required to supplement the oxygen already dissolved in the molten steel to facilitate the smooth and expectable production of ultra-low carbon steel. Owing to the addition of manganese ore to increase the oxygen content of the molten steel, a broader variety of initial molten steel conditions can be successfully treated by the RH process and, more particularly, a steel having a higher carbon content or lower oxygen content can be decarburized to ultra-low levels in a relatively short treatment time. This is extremely important in that it increases the rate at which the optimum vacuum treatment is obtained and partially releases the blowing burden of the basic oxygen furnace or Q-BOP, resulting in increased steelmaking productivity and furnace availability.
This invention is also applicable to some vacuum decarburization processes other than those employing an RH vacuum degasser, for example, an electrical refining furnace equipped with vacuum treatment.